Carrier AERO 39MN Installation, Start-Up And Service Instructions Manual page 105

Each heater has 2 different types of factory-installed ther-
mal cutouts for over temperature protection; an automatic re-
set thermal cutout for primary protection and a manual reset
thermal cutout to protect against failure of the primary sys-
tem. Also provided is an airflow pressure differential switch
to prevent the heater from operating when the fan is not in op-
eration or airflow is restricted or insufficient. The primary au-
tomatic reset cutout is a bi-metal disk-type cutout. It is wired
into the control circuit which operates the magnetic discon-
necting contactors (the same contactors which also switch on
and off the various steps of the coil). The secondary manual
reset cutout is a bi-metal disk-type cutout. This secondary
thermal cutout is load carrying and is installed in each heater
subcircuit. The primary and secondary overtemperature pro-
tection systems are independent of each other. The secondary
system is designed to protect against possible failure of the
primary system to deenergize the heater.
Subcircuits in the heaters are designed in compliance with
paragraph 424-22 of the NEC. The coil is subdivided into
circuits that draw no more than 48 amps each and is fused for
at least 125% of the circuit rating.
NOTE: Airflow tube is to be positioned so that the airflow
switch is actuated by a minimum negative pressure of 0.07
in. wg. In the event that the minimum 0.07 in. wg cannot be
achieved, extend the high pressure tube to the next upstream
bulkhead to the fan discharge for draw-thru electric heat or
extend the low pressure tube to the negative side of the fan
for blow-thru electric heat.
Refer to Table 31 for heater electrical data.
a39-1721
NOTE: All wiring must be copper and must conform to the NEC (National Electrical Code).
Fig. 90 — Electrical Heater Wiring Schematic (Typical)
Table 30 — Field Wiring for Incoming Conductors
Sized for 125% of Heater Load
LOAD AMPS*
WIRE SIZE
(AWG or kcmil)
Copper
14 12
12 16
10 24
8 40
6 52
4 68
3 80
2 92
1 104
1/0 120
LEGEND
AWG — American Wire Gage
kcmil — Thousand Circular Mils
NEC — National Electrical Code
*Values are based on Table 310-16 of the NEC for 75 C insulated cop-
per wire. Not more than 3 conductors in a raceway.
NOTES:
1. Be sure to consider length of wiring run and possible voltage
drops when sizing wires.
2. Field power wiring — Heaters are furnished with a terminal block
sized for incoming copper conductors with 75 C insulation rated
to carry at least 125% of the heater load. However, conductors
can be sized to carry 100% of the heater load if the heater is
rated at 50 kW or more, and the heater is controlled by a cycling
device such as a multi-stage thermostat, step controller, or SCR
(silicon control rectifier) power controller. Terminal blocks and
knockouts are sized to handle either 100% or 125% conductors.
104
LOAD AMPS*
WIRE SIZE
(AWG or kcmil)
Copper
2/0 140
3/0 160
4/0 184
250 204
300 228
350 248
400 268
500 304
600 336
700 368